In the stator of a gas inner-cooled electrodynamic power generator, such as a multiphase electrical generator of the type manufactured by the assignee of the present invention, stator coils are constructed of columns or stacks of copper strands and associated stacks of vent tubes through which a coolant such as hydrogen gas may be circulated. Such generators commonly incorporate a plurality of stator coils, which are the high voltage AC winding armature elements that provide the generator's output voltage and current. Stator windings are formed of conductive stator coils in which AC voltage is induced by the passage of the rotating magnetic flux field generated by the generator rotor. Each stator winding may commonly include a plurality of half-coils where each half-coil extends the length of the slot in the stator and is joined to another half-coil or to a phase end lead at the end turn or involute portion of the stator assembly. The half-coil may include two stacks of copper conductor strands and one or more stacks of vent tubes, or other arrangements. The individual copper strands in adjacent strand stacks are transposed to form a pair of braid like roebel bars to reduces losses from cross slot flux and to reduce hot spot temperatures. Such coil arrangements are commonly referred to as single or double tube stack inner-cooled stator coils. Other coil arrangements, such as those using water as the coolant, are known in the art and are similarly constructed.
In the stator coil ends or involutes, the effect of voltage induced by end region flux is conventionally controlled by known connections referred to as a group transposed series connection. Such a connection joins the conductive strands from a first half-coil to corresponding strands in a second half-coil by separating the strands into individual strand groups. Each of the strand groups is then joined with a corresponding strand group in an individual series connection. This process is labor-intensive, time consuming and cumbersome. First, the individual strands must be separated and their strands regrouped into specified bundles. The strands must also be cleaned, tinned, bundled into clips, and soldered into the series connector. The purpose of tinning is to provide a uniform solder coating that will prevent voids when the strands are bundled into crimped or bolted connectors and heat fused together. The exposed strand ends are first cleaned with an abrasive wheel or by hand and are then wiped with alcohol to remove dust and other contaminates. Next, the strands are brushed with a rosin alcohol flux to prepare the copper surface for tinning. The copper strand ends are then hand dipped into heated solder. Excess solder is allowed to drip off and is smoothed by wiping. An alternative method, which is more reliable but far more laborious, is to apply the solder coating by hand, to each strand individually, using a soldering iron with a thermocouple attached to control soldering temperature.
Either method is time consuming, arduous and requires considerable skill and care to achieve uniform tinning. A deviation from precise temperature/time process requirements produces weak and uneven adhesion between the copper and the solder. Lumps and irregular thickness of solder may also be produced. When these strands are fitted into a bolted series or phase connector, their uneven coating may prevent effective tightening. Cold solders may cause voids to develop between strands, which can cause the unit to fail ultrasonic inspection. In that case, the entire process must be done over. Further, while tinning is usually done on-site, it is sometimes performed at the stator coil manufacturing facility prior to shipment of the coil to a site. However, the tinning process may be repeated on-site due to damage during shipment and/or the need to meet on-site specifications to ensure a proper conductive connection between the half coils. The working space for on-site tinning and connecting the half coil ends is often cramped, which may lead to an inferior connection. The cost of on-site rework, delays and making inferior connections between half coils can be considerable.
Furthermore, retrofitting and/or providing other maintenance services on existing electric generators having such transposed connections are time consuming and expensive. For example, a series connection transposition may short due to aging insulation surrounding part of the connection. In this respect, determining the location of the short and repairing and/or replacing series transposed connections related to the short is labor intensive, causing significant downtime of the generator, which may lead to a significant loss of revenue for the operator.
A solid series connector is disclosed in U.S. Pat. No. 5,270,598 that includes a first and second conductor clip where each clip is brazed only to an outer stack of conductive strands in a single tube stack inner-cooled half coil. A braze alloy may be placed on the outer surfaces of the stacks for brazing the conductor clips to the outer stacks. Conductor members may be brazed to the outer surfaces of the conductor clips to conductively connect a pair of half coils. It has been determined that removal of the conductor members for maintenance and/or repair of a half coil may disturb the consolidation of the conductive strands within each stack, which may lead to costly repairs.